It is now commonplace to provide security threads within bank notes and other security documents. Originally a security thread was a simple metallic thread, but-according to current practice the thread, which may typically comprise a laminate of long, thin strips of polyester and an intermediate magnetic track, is magnetically coded. The magnetic coding of the strip is preferably digital, usually binary, and it is usual for the magnetic track to be constituted by a succession of segments each associated with a particular digital value, the length of the segments or `bit lengths` being constant, typically about 2 mm. Examples of magnetic threads of this kind are disclosed in British Patent GB-B-2098768 and European Patent EP-B-0407550. Other magnetic threads with which the invention may be used are disclosed in EP-B-0310707 and EP-A-0428779. The particular manner of representation of the different digital values may vary. For example, in the specific example described in EP-B-0407550, a segment which represents a `0` is characterised by the absence of magnetic material and a segment which represents a binary `1` is constituted by the presence of magnetic material. Thus the track may be constituted by lengths comprising one or more segments of magnetic material or segments having no magnetic material, depending on the digital coding of the thread. However, the invention is not limited to use with such a thread; other forms of representation of the digital values, such as different thicknesses of thread, different widths of thread or other different magnetic characteristics, can be employed, provided that a reader possibly including a threshold detector can distinguish between the characteristics of different segments. It may be noted that there is not necessarily an identifiable boundary between adjacent segments, though proper framing of the digital output can be achieved by the use of termination codes between the code words.
It has hitherto been considered necessary to read a magnetic track by means of a detector assembly with respect to which the thread is moved with a substantial component of movement along the length of the track. Such a state of the art is acknowledged, for example, in EP-A-0413534. This is a substantial disadvantage, because it is customary to provide for a bank note or other security document, which is rectangular in form, a security thread which extends across the document from top to bottom so that the thread is parallel to the shorter sides of the document. Accordingly, if the thread has to be read longitudinally, the document must be fed such that the longer side is presented first.
EP-A-0413534 describes an attempt to alleviate this difficulty and describes a reader for a note that has the security thread running from top to bottom parallel to the shorter side and is presented to a reader with a shorter side leading. The solution proposed in EP-A-0413534 is the use of a detector which extends at a substantial acute angle, typically 45.degree., to the line of movement of the bank note, this line of movement being at right angles to the security thread. This is an awkward and rather inefficient solution.